1. Field of the Invention
The present invention relates to a technique of varying a compression ratio in an internal combustion engine. More specifically the invention pertains to a technique of detecting a failure or trouble arising in a compression ratio varying mechanism and controlling an internal combustion engine.
2. Description of the Related Art
The internal combustion engine is advantageously small in size but is capable of outputting relatively large power. Because of these advantages, the internal combustion engine is widely used as the power source of diverse transportation facilities, such as automobiles, ships and boats, and aircraft, and as the power source of various stationary machines and equipment. The internal combustion engine makes a compressed air-fuel mixture subjected to combustion in a combustion chamber, converts the combustion pressure generated through the combustion into mechanical power, and takes out the mechanical power.
A technique of varying the compression ratio of the air-fuel mixture according to the driving conditions of the internal combustion engine has been proposed to improve a conversion efficiency into mechanical power (that is, a thermal efficiency) and increase an maximum output. Setting a low compression ratio ensures a sufficiently high maximum output under the conditions of high loading. Setting a high compression ratio enhances the thermal efficiency under the conditions of medium or low loading. The optimum ignition timing depends upon the compression ratio. The lower compression ratio advances the optimum ignition timing. The general control procedure thus changes the ignition timing with a variation in compression ratio.
One proposed technique fixes the ignition timing to the setting suitable for the high compression ratio, when any failure arises in the course of varying the compression ratio in the internal combustion engine (see Japanese Patent Laid-Open Gazette No. 1-35047). The technique of this cited reference fixes the ignition timing to the setting for the high compression ratio, when the compression ratio is locked in at the high compression ratio. The technique prevents the ignition timing from advancing to the setting suitable for the low compression ratio, in the case of a lock-in of the compression ratio, thus lowering the potential for abnormal combustion called knocking.
The above prior art technique lowers the potential for knocking but may not allow the internal combustion engine to be driven stably, when some trouble or failure arises in the course of varying the compression ratio. Even when the compression ratio is locked in at the low compression ratio, this proposed technique fixes the ignition timing to the setting suitable for the high compression ratio. Such fixation is disadvantageous to stable driving of the internal combustion engine. The low compression ratio is undesirable for quick and stable combustion of the air-fuel mixture in a combustion chamber. For stable combustion of the air-fuel mixture, the ignition timing is thus to be changed to the adequate setting with a decrease in compression ratio. Diverse controls are carried out in the internal combustion engine with the aim of improving the thermal efficiency or of reducing the emission. Some of such controls have adverse effects on stable combustion. Execution of such controls in the state of a lock-in of the low compression ratio may lower the stability of combustion and prevent the internal combustion engine from being driven stably.